Pretty well as everybody else mentions, look to energy food and fluids to keep the body running right. I personally find that the mental challenge is a big factor; the bike's right, your fitness is right, now tell yourself you can do it. I also have to school myself not to start the ride like a greyhound from the traps, rather start at a steady pace so I've got something left for the last few miles. A couple of the mental games I play is to use the cycle computer to carve the ride up into smaller portions in my mind. You did the first ten easy enough, the next ten will be easy too right? I also use my bag of sweet nibbles to reward myself for another ten miles done, or getting to the top of this hill without stopping, or any other mini goal of your choice.
Whatever, it's a buzz when you do it!